This invention generally relates to the field of inverter circuits and, more particularly, to inverter circuits that illuminate electroluminescent lamps.
Because of their compact size and low current consumption, electroluminescent lamps are widely used in small electronic devices. For example, such lamps are used for backlighting liquid crystal displays in portable communication devices, such as cellular telephones. Essentially, an electroluminescent lamp is a capacitor with a phosphorous dielectric. The lamp emits light when it is excited by applying a sufficiently high AC voltage across its electrodes. To emit light continuously, the lamp must be charged and discharged at a low frequency during successive charge and discharge cycles. For this reason, the drive signal for the electroluminescent lamp is a high-voltage, low-frequency AC drive signal. Depending on the size of the electroluminescent lamp and the desired illumination intensity, this signal can have a voltage level in the range of 100-150 volts and a frequency in the range of 100-400 Hz.
Generally, a DC power supply generating a DC supply voltage in the range of 1-5 volts powers the electronic device in which the electroluminescent lamp is used. Such a voltage is significantly lower than the voltage level required to illuminate the lamp. Therefore, an electronic device that uses electroluminescent lamp typically includes an inverter circuit which converts the low DC supply voltage to a high-voltage, low-frequency AC drive signal.
Various types of inverter circuits have been used in the past to convert a DC supply voltage to an AC drive signal. One known inverter circuit uses a transformer; transformers, however, are bulky and expensive, and thus are not suited for use in small electronic devices. Bridge inverter circuits, which include a DC diagonal and an AC diagonal, have also been used for driving electroluminescent lamps. In a bridge inverter circuit, a DC diagonal couples to the DC power supply, and an AC diagonal couples across the electroluminescent lamp. The diagonals increase the DC supply voltage and provide an alternating current to the electroluminescent lamp. The bridge inverter circuit further includes level shifters for operating a set of high-side switches. These level shifters add significantly to the cost and complexity of the electronic device.
Another conventional approach uses an inverter pump circuit in which the energy stored in an inductor is switched at high speed to produce a series of high voltage pulses. These pulses successively charge the electroluminescent lamp to a sufficiently high voltage level during a charge cycle. One of the advantages offered by such an inverter circuit is that by increasing the switching frequency, the size of the inductor can be reduced, thereby reducing the size of the inverter circuit. During a discharge cycle, the electroluminescent lamp is discharged through a short circuit. The short circuit is coupled across the lamp during the discharge cycle by closing a switch which is open during the charge cycle. By closing and opening the switch at a low frequency, the inverter pump circuit produces a low-frequency AC drive signal across the electroluminescent lamp. Generally, a control logic controls the switch during the charge and discharge cycles.
During the discharge cycle, however, a conventional inverter pump circuit produces humming noise components that are irritating to the human ear. More specifically, during the discharge cycles, the AC drive signal produced by this type of inverter circuit has steep voltage transitions, which are caused by the sudden discharge of the electroluminescent lamp through the short circuit. These sudden discharges produce various undesired frequency components, including the undesired humming noise components. Therefore, there exists a need for a simple and inexpensive inverter circuit for illuminating electroluminescent lamps in small electronic devices, which does not produce the irritating humming noise components.